PIM Dense Mode

PIM has two modes of operation ‚ dense mode and sparse mode. Dense mode uses a flood-and-prune mechanism to forward multicast packets. The router assumes that every multicast interface is interested in multicast packets, unless it is told otherwise . The router first forwards multicast packets to all the interfaces. Segments that don't want multicast packets receive prune messages from the neighboring routers, and the branch is pruned.

When the router is first configured for multicast, the router sends periodic PIM query packets to the multicast address of 224.0.0.2 (all routers on this subnet address) to discover its PIM neighbors. The PIM query packets are sent out the interfaces that are configured for PIM. PIM neighbors are established across an interface when PIM queries are received on that interface.

PIM dense mode floods multicast packets on its out interface list (also known as an oilist ). PIM dense mode puts an interface in its oilist if the following conditions are true:

• The interface has an established PIM neighbor.

• The interface has hosts joining the multicast group through IGMP.

• The interface has been manually configured to join the group through the ip igmp join-group command.

When a router running PIM dense mode first receives multicast packets, it floods the multicast packets to all interfaces listed in the oilist. The router stops forwarding multicast packets on an interface if it receives a prune packet from its neighbor.

In Figure 12-6, the Router R1 receives incoming multicast packets on interface S0. As R1 is running dense mode, it floods the multicast packets to all its oilist interfaces, E0 and S1. Because Router R2 doesn't have any hosts interested in multicast traffic, it sends a PIM prune message toward R1. When R1 receives the PIM prune, it waits for three seconds before it stops forwarding multicast packets for the group to interface E0. This three-second delay allows other routers on the segment to override the prune with a PIM join.

Figure 12-6. PIM Dense-Mode Pruning

When the interface has been pruned, the router stops forwarding multicast packets for the group to the interface. In Figure 12-6, suppose that Router R2 now has a host that wants to receive multicast packets on interface E1. Router R2 sends a PIM graft packet to Router R1. When Router R1 receives the PIM graft message, it puts its interface E0 into a forwarding state, and multicast traffic flows to Router R2.

If there are two routers on the same LAN interface and both routers have a connection to the source of the multicast stream, both routers potentially could forward multicast packets and cause duplicate packets on the LAN interface. PIM dense mode has an assert mechanism to avoid such scenarios. Figure 12-7 illustrates the PIM assert mechanism.

Figure 12-7. PIM Assert Mechanism

In the absence of the PIM assert process, both routers forward multicast packets onto interface E0. This causes duplicate packets on the LAN interface. With the assert mech-anism, both routers send the PIM assert packet with the unicast routing distance and metric to the source of the multicast stream. The router with the best unicast route to the source wins and starts forwarding the multicast packets. The router that loses the assert battle prunes the outgoing interface. If a tie is on the unicast routing metric, the router with the highest IP address wins the assert. This way, only one router is actively forwarding the multicast traffic.